This invention relates to stress gauges, and more particularly to free field stress gauges capable of dynamic or static responses up to and including the kilobar (nominally 15,000 p.s.i.) range.
Free field stress gauges as defined herein are gauges that are designed to be physically imbedded in or surrounded by a given body of material (medium) and are used to provide a measurement of any stress in the medium in which they are being used. Load cells, on the other hand, which are common well known stress gauges are generally used to measure the force generated by or the weight of a given body. Further, the physical properties of a load cell may be independent of the medium of interest, which is not true for an ideal free field stress gauge.
An ideal free field stress gauge has the same deformation and density characteristics as the surrounding medium. Furthermore, the gauge should have constant deformation characteristics across its entire face. If these conditions do not exist and the density and deformation characteristics of the stress gauge are allowed to vary in an uncontrolled manner, the stress gauge, especially in the dynamic free field mode, could disrupt the internal stress field of the medium to such an extent that data generated by the gauge would be erroneous. If significant density mismatch occurs, dynamic response of the stress guage and surrounding medium will differ and the gauge output will suffer. Similarly, if the stiffness or resistence to deformation (Modulus) of the stress gauge is less than that of the medium, the indicated or measured stress would be less than the true stress in the medium. The converse is also true, particularly in granular soil and rock media.
An ideal free field stress gauge should also have uniaxial sensitivity. It should sense stress in only one direction. This is required because materials which possess shear strength will have, under certain conditions of multicomponent loading, an internal stress distribution which is not equal in all directions. The only way to observe the actual internal state of stress for such a material in such a loading condition would be to embed a multiplicity of uniaxial stress gauges into the material in such a way that their sensitive axes were oriented in different directions.
The invention provides a free field stress gauge that is so designed that effective control of gauge density (weight per unit volume) and deformation (modulus) is provided. Uniaxial sensitivity is also achieved. Further, this invention provides a free field stress gauge suitable for use in stress fields up to and including the kilobar range.